Catalytic cracking is an established and widely used process in the petroleum refining industry for converting petroleum oils of relatively high boiling point to more valuable lower boiling products, including gasoline and middle distillates, such as kerosene, jet fuel and heating oil. The preeminent catalytic cracking process now in use is the fluid catalytic cracking process (FCC) in which a preheated feed is brought into contact with a hot cracking catalyst which is in the form of a fine powder, typically having a particle size of about 10-300 microns, usually about 100 microns, for the desired cracking reactions to take place. During the cracking, coke and hydrocarbonaceous material are deposited on the catalyst particles. This results in a loss of catalyst activity and selectivity. The coked catalyst particles, and associated hydrocarbon material, are subjected to a stripping process, usually with steam, to remove as much of the hydrocarbon material as technically and economically feasible. The stripped particles containing non-strippable coke, are removed from the stripper and sent to a regenerator where the coked catalyst particles are regenerated by being contacted with air, or a mixture of air and oxygen, at elevated temperature. This results in the combustion of the coke which is a strongly exothermic reaction which, besides removing the coke, serves to heat the catalyst to the temperatures appropriate for the endothermic cracking reaction. The process is carried out in an integrated unit comprising the cracking reactor, the stripper, the regenerator, and the appropriate ancillary equipment. The catalyst is continuously circulated from the reactor or reaction zone, to the stripper and then to the regenerator and back to the reactor. The circulation rate is typically adjusted relative to the feed rate of the oil to maintain a heat balanced operation in which the heat produced in the regenerator is sufficient for maintaining the cracking reaction with the circulating regenerated catalyst being used as the heat transfer medium. Typical fluid catalytic cracking processes are described in the monograph Fluid Catalytic Cracking with Zeolite Catalysts, Venuto, P. B. and Habib, E. T., Marcel Dekker Inc. N.Y. 1979, which is incorporated herein by reference. As described in this monograph, catalysts which are conventionally used are based on zeolites, especially the large pore synthetic faujasites, zeolites X and Y.
Typical feeds to a catalytic cracker can generally be characterized as being a relatively high boiling oil or residuum, either on its own, or mixed with other fractions, also usually of a relatively high boiling point. The most common feeds are gas oils, that is, high boiling, non-residual oils, with an initial boiling point usually above about 230.degree. C., more commonly above about 350.degree. C., with end points of up to about 620.degree. C. Typical gas oils include straight run (atmospheric) gas oil, vacuum gas oil, and coker gas oils.
While such conventional fluid catalytic cracking processes are suitable for producing conventional transportation fuels, such fuels are generally unable to meet the more demanding requirements of low emissions fuels. To meet low emissions standards, the fuel products must be relatively low in sulfur, nitrogen, and aromatics, especially multiring aromatics. Conventional fluid catalytic cracking is unable to meet such standards. These standards will require either further changes in the FCC process, catalysts, or post-treating of all FCC products. Since post-treating to remove aromatics from gasoline or distillate fuels is particularly expensive, there are large incentives to limit the production of aromatics in the FCC process. Consequently, there exists a need in the art for methods of producing large quantities of low emissions transportation fuels, such as gasoline and distillates.